When a ray of light passes through a photoelastic material, its electromagnetic wave components are resolved along the two principal stress directions and each component experiences a different refractive index due to the birefringence. The difference in the refractive indices leads to a relative phase retardation between the two components. Assuming a thin specimen made of isotropic materials, where two-dimensional photoelasticity is applicable, the magnitude of the relative retardation is given by the stress-optic law Δ=((2πt)/λ)C(σ₁-σ₂), where Δ is the induced retardation, C is the stress-optic coefficient, t is the specimen thickness, λ is the vacuum wavelength, and σ₁ and σ₂ are the first and second principal stresses, respectively.
When a ray of light passes through a photoelastic material, its electromagnetic wave components are resolved along the two principal stress directions and each component experiences a different refractive index due to the birefringence. The difference in the refractive indices leads to a relative phase retardation between the two components. Assuming a thin specimen made of isotropic materials, where two-dimensional photoelasticity is applicable, the magnitude of the relative retardation is given by the stress-optic law $\Delta ={\frac {2\pi t}{\lambda }}C(\sigma _{1}-\sigma _{2})$, where $\Delta$ is the induced retardation, $C$ is the stress-optic coefficient, $t$ is the specimen thickness, $\lambda$ is the vacuum wavelength, and $\sigma_1$ and $\sigma_2$ are the first and second principal stresses, respectively.
Applicable units are those of quantitykind:StressOpticCoefficient
Stress-Optic Coefficient